The earliest ice skating rinks were frozen ponds or lakes. Such ice sport venues had the sizeable limitation that their existence was entirely dependent upon the temperature of the environment. For a long time, the dependency upon naturally-formed ice restricted the enjoyment of ice sports in most countries to a limited seasonal period.
In the late nineteenth century, indoor ice skating rinks were designed to provide venues on which ice sports could be enjoyed in most countries year-round. These early indoor ice skating rinks used a system of steel or iron pipes to carry an artificially-cooled refrigerant, such as calcium chloride brine, under a tank of water to create a frozen surface capable of being skated upon. The steel or iron pipes were embedded in concrete or sand beneath the tank, and had an inner diameter of 1 to 11/2 inches with 4 inches between the centers.
While capable of providing a frozen surface which could be skated upon indoors year-round, the steel or iron pipe construction had its drawbacks. Perhaps, one of the greatest limitations on the steel or iron constructions was the surface area that these systems provided for heat exchange with the medium to be frozen, also known as the dynamic surface area. In the steel or iron constructions, as structurally and dimensionally described above, the dynamic surface area was substantially less than the area of the skating surface available for heat exchange with the environment. The dynamic surface area of the steel or iron constructions is estimated to be at most 82% of the skating surface area.
More recently, ice skating rink systems have been constructed using smaller diameter plastic tubing, such as those systems described in U.S. Pat. Nos. 3,751,935; 3,893,507; and 3,910,059. In operation, a main supply pipe, or header, feeds into a plurality of supply subheaders, each of which in turn is attached to the proximal ends of a plurality of coolant tubes. The plurality of coolant tubes can be fastened at their distal ends to one end of a plurality of U-shaped connectors, which in turn are fastened to a second plurality of coolant tubes. The second plurality of coolant tubes is attached at their proximal ends to a plurality of return subheaders, which in turn feed into a main return header. The inner diameter of the coolant tubes used in these plastic constructions generally varies from 1/4 to 1/2 inches. By using a smaller center spacing between smaller tubes, these plastic systems may provide a larger dynamic surface area than the steel or iron constructions.
However, the dynamic surface area is only one factor influencing the overall efficiency of a system designed to create and maintain a frozen surface. As important to the efficiency of the system as the dynamic surface area is the ability of the coolant to flow through the system without significant pressure loss or flow interruption. As a consequence, even though the plastic systems may have improved the dynamic surface area over the iron and steel constructions, the efficiency of these plastic systems is often significantly compromised in practice by unsatisfactory coolant flow characteristics at various points in the system.
For example, as shown in FIGS. 1 and 2 herein, one common area for flow restriction to occur is at the transfer point between a subheader 30 and a coolant tube 32. In the conventional construction shown in FIGS. 1 and 2, the subheader 30 has an opening 34, through which is disposed a connection fitting 36. The connection fitting 36 is soldered into place with the proximate end of the fitting 36 occluding as much as 25 percent of the interior cross-sectional area of the subheader 30. This occlusion can cause a layer 38 of coolant to build up against the fitting 36, and seriously degrade the flow characteristics of the coolant in the area adjoining the transfer point.
Moreover, at the distal end of the tube 32, where the tube 32 attaches to a U-shaped connector 40, the conventional methods of construction can cause additional flow restriction problems. One flow restriction problem commonly occurring in conventional constructions is illustrated in FIGS. 3 and 4. The U-shaped connector 40 shown is fabricated by bending a copper tube having an internal diameter similar to that of the coolant tube 32. By using this method of fabrication, the resulting inner diameter at a bight 42 of the U-shaped connector 40 may be reduced to approximately half the diameter of the original copper tube. The dramatic decrease in the inner diameter of the U-shaped connector 40 at the bight 42 has a proportionally dramatic effect on the fluid flow throughout the system.
Additionally, loss of flow pressure can result from the present methods of system construction used to join the coolant tubes 32 with the U-shaped connectors 40. The coolant tubes 32 are fastened directly to the U-shaped connectors 40 by means of glue and a circular clamp or an eyelet, as shown in FIGS. 3 and 4. As a consequence, the tubes 32 have a tendency to leak, or even pop off of the U-shaped connector 40, spilling coolant directly into the medium to be frozen and underlying foundational material and decreasing the pressure and flow rate at which the coolant is being transported throughout the system.
Furthermore, these plastic systems are often constructed using a type of plastic coolant tube having unfavorable performance characteristics. Commonly, polyethylene or polypropylene tubing is used for the coolant tubes in plastic ice skating rink systems. During manufacture, the polyethylene or polypropylene tubing is usually extruded, and then passed through a standard length (10-14 foot) cooling tank before being machine-coiled on to spools for delivery. As a consequence of this method of fabrication, the polyethylene or polypropylene tubing thermally sets with a curved, rather than a straight, structure in the memory of the plastic. Therefore, when the tubing is uncoiled to be used in the plastic construction illustrated in the patents mentioned above, the tubing does not naturally lay straight and flat, but takes on a serpentine structure in at least one plane.
As a further consequence, when these polyethylene or polypropylene ice rink systems are installed, the coolant tubing will commonly force its way under pressure to the skating surface, and protrude from the surface of the ice, providing a substantial obstacle and hazard for persons, for example skaters, using the frozen surface. It is therefore necessary to resubmerge the tubing under the surface of the ice through a method known as "burning in". The tubing is "burned" into the surface of the ice by melting the surrounding ice, and then holding the tube in place under pressure until the ice reforms around the problematic section of tubing. Because of the pressure of the coolant running through the tubing, as well as the thermally-set disposition of the tubing to return to the serpentine structure, it may be necessary to repeat the "burning in" process a number of times each season to maintain a skating surface free from obstructions and to prevent damage to the tubing.
However, polyethylene and polypropylene tubing is sensitive to repeated bending. Repeated bending of the polyethylene or polypropylene tubing has been known to cause permanent damage to the tubing, and can result in the cracking or rupture of the tubing with a concomitant loss of coolant pressure in the system.